Question about 1994 GMC Jimmy

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1994 jimmy 4.3 vortec has a stumble right off of idle when accelerating and a hesitation once and awhile at random spaads ussually under load any suggestions?

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  • cindyryan68 Jul 22, 2010

    does it hot or cold

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  • GMC Master
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Do you have an O2 sensor code that would explain a random miss?
Does it happen cold or hot? Have you checked the spark from the coil?

Diagnosis Before beginning any diagnosis and testing procedures, visually inspect the components of the ignition system and engine control systems. Check for the following:

  • Discharged battery
  • Damaged or loose connections
  • Damaged electrical insulation
  • Poor coil and spark plug connections
  • Ignition module connections
  • Blown fuses
  • Damaged vacuum hoses
  • Damaged spark plugs
Check the spark plug wires and boots for signs of poor insulation that could cause crossfiring. Make sure the battery is fully charged and that all accessories are off during diagnosis and testing. Make sure the idle speed is within specification. If an open or ground in the Ignition Control (IC) circuit occurs during engine operation, then engine will continue to run, but using a back-up timing mode (controlled by the ICM) based on preset timing values. The Malfunction Indicator Lamp (MIL) or SERVICE ENGINE SOON light will not illuminate at the first appearance of a break in the circuit. However, if the IC fault is still present once the engine is restarted, a Code 42 will set on OBD 1 systems in the PCM and the MIL will illuminate. Poor performance and fuel economy may be noticed while the engine is running under back-up timing. When attempting to search for ignition troubles, keep in mind the various sensor inputs which the PCM uses to calculate timing may affect engine performance. The PCM will alter timing based on sensor inputs as follows:
  • Low MAP output voltage = More spark advance
  • Cold engine = More spark advance
  • High MAP output voltage = Less spark advance
  • Hot engine = Less spark advance
With this in mind, DETONATION could be caused by low MAP output or high resistance in the coolant sensor circuit. POOR PERFORMANCE could be caused by a high MAP output or low resistance in the coolant sensor circuit.

Posted on Jul 22, 2010

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